Ultrafast Circuits

The current research activities carried out by the NDR group in this field mainly cover two aspects of resonant-tunneling diode circuit applications, the multi-valued logic and the ultrafast digital circuits.

Multivalued Logic RTD-based Circuits

The nonlinear tunneling characteristics of resonant-tunneling diodes can be efficiently harnessed to design multiple-valued circuits requiring fewer active devices and less amount of connecting wires between them. Interconnects will predominantly govern the circuit speed in future gargantuan multi-billion transistor monolithic integrated chips. Multiple-valued logic (MVL) can alleviate the interconnect delay and routing complexities since multi-valued signals convey more information than binary signals, thus requiring less amount of interconnects to transmit similar bandwidth of information. Resonant-tunneling devices can be stacked together vertically to develop multistate memories and multi-valued logic gates.

We have developed the following RTD-based multi-valued logic circuits at The University of Michigan:

• Multi-valued MUX/DEMUX circuit
  [ E. Chan, S. Mohan, P. Mazumder and G. I. Haddad, "Multivalued Multiplexer Design Using Resonant Tunneling Devices and Heterojunction Bipolar Transistors," IEEE Journal of Solid-State Circuits, vol. 31, no. 8, pp. 1151-1156, August 1996. ]
• Multivalued logic programmable gate array
  [ E. Chan, M. Bhattacharya, P. Mazumder and G.I. Haddad, "Mask Programmable Multi-Valued Logic Gate Arrays Using RTDs and HBTs," IEE Proceedings-E: Computers and Digital Techniques, Vol. 143, No. 5, pp. 289-294, October 1996. ]
• Three-valued signed-digit full adder circuit
  [ A. F. González and P. Mazumder, "Multiple-Valued Signed-Digit Adder Using Negative Differential-Resistance Devices," IEEE Transactions on Computers, vol. 47, no. 9, pp. 947-959, September 1998. ]

High-Speed Digital RTD-Based Circuits

Resonant-tunneling diodes have been demonstrated at frequencies in excess of 700 GHz, they can operate at room temperature, and they can easily be co-integrated with high-speed active devices such as heterojunction bipolar transistors (HBTs), and high-electron mobility transistors (HEMTs). Due to their nonlinear current-voltage characteristics, resonant-tunneling devices are inherently bistable: they are capable of retaining the output states after the input signals change their values. This feature allows each logic gate to work as a bistable latch, which enables the development of deeply pipelined circuits (what is termed nanopipelining) capable of achieving much higher speed and throughput. The objective of our work in this area is to employ the high-speed capabilities of RTDs in ultrafast digital circuits for applications where such performance is required. These applications include digital signal optical transmission systems and high-speed digital signal processing.

Currently, our group is developing ultrafast latches and flip-flops and digital multiplexing and demultiplexing circuits.